1. Field of the Invention
The present invention relates to an optical waveguide device including an optical waveguide element having an optical waveguide formed in the surface of a dielectric substrate, and a package housing accommodating the optical waveguide element.
2. Description of the Related Art
An optical waveguide element having an optical waveguide formed in the surface of a dielectric crystal substrate of lithium niobate (LiNbO.sub.3), for example, by thermal diffusion of titanium (Ti) or its oxide, for example, has features including low loss, wide band, and mass productivity using a planar technique, and has been widely researched and developed to become practical. In particular, attention is focused on a Mach-Zehnder (branch interference type) modulator chip owing to its high-speed performance, which is configured by forming an optical waveguide and a plurality electrodes on a dielectric crystal substrate. By controlling a voltage to be applied to the electrodes, the refractive index of the optical waveguide is adjusted to thereby perform a switching operation and a modulating operation.
The Mach-Zehnder modulator chip is configured by forming an optical waveguide consisting of a pair of arm portions connected together at their opposite ends in the surface of a dielectric substrate formed of a Z-cut lithium niobate crystal, forming a buffer layer of SiO.sub.2 on the optical waveguide, and forming a plurality of metal electrodes (signal electrode and ground electrode) so as to correspond to the optical waveguide. The optical waveguide is formed by thermal diffusion of titanium into the surface of the dielectric substrate to thereby increase the refractive index.
Light entering the optical waveguide from its one end is branched into two beams to reach the pair of arm portions of the optical waveguide. At this time, a drive voltage is applied to one of the electrodes, so that a phase difference is produced between the two beams in the two arm portions by the electro-optical effect (the directions of electric fields at the two arm portions are opposite to each other, so that the refractive indices of the two arm portions change in opposite directions). These two beams are combined into one beam at the other end of the optical waveguide, and this beam is taken out as an optical signal output. By applying the drive voltage so that the phase difference between the two beams becomes zero or .pi., an ON-OFF digital signal, for example, can be obtained.
The modulator chip as mentioned above is formed in the shape of elongated rectangular rod having dimensions of about 1.times.1.times.60 (mm), for example, and it is accommodated in a package housing formed of metal for the purposes of connection with an optical fiber of the like and protection, thus configuring an optical waveguide device. The package housing has an element mounting surface having a grounding portion to be connected to the ground electrode of the modulator chip. An element inserting groove slightly larger in size than the optical waveguide element is formed on the element mounting surface.
The optical waveguide element is fixed to the package housing by applying an adhesive to the inner surface (bottom surface) of the element inserting groove with an adhesive applicator rod and thereafter inserting the optical waveguide element into the element inserting groove. Thereafter, the ground electrode of the optical waveguide element and the grounding portion of the element mounting surface are electrically connected together by ribbon (wire) bonding. Further, the signal electrode of the optical waveguide element is electrically connected to a voltage applying terminal insulated from the package housing.
In the conventional optical waveguide device as mentioned above, the element inserting groove formed on the element mounting surface of the package housing is very thin (narrow) as having a width of about 1 mm and a depth of about 1 mm. Accordingly, the work of applying the adhesive is very difficult to carry out, so that there is a possibility that the adhesive may be applied in an excess amount or may be erroneously applied also to the side surface of the element inserting groove or other portions. Further, a gap between the opposed side surfaces of the optical waveguide element and the element inserting groove is narrow. Accordingly, in the case of erroneous application of the adhesive to the side surface of the element inserting groove or application of the adhesive in an excess amount as mentioned above, the adhesive may rise in this gap by capillarity to reach the element mounting surface, causing adverse effects on characteristics (high-frequency characteristics), reliability, and bonding ability of the optical waveguide element.